To produce accurately positioned duplex (two sided) images, whether by a printer or copier, the front side and rear side images are usually referenced from a same edge of a sheet on which they are printed. Since many inverters invert the sheet so the leading edge (from which the front image is referenced), becomes the trailing edge and since most printers reference the current leading edge, the rear side imager lacks a reference to the image on the front side.
Some prior art duplex imaging systems use relatively complex measurement systems to determine the position of the current trailing edge and use that edge as a reference for the printing of the rear image. Other prior art systems use bulky and/or complex mechanisms to rotate an inverted sheet to restore its reference edge to the lead position; one such system comprising an arm that grabs the sheet, rotates the arm 180 degrees about an end of the arm remote from the sheet and releases the sheet.
A skewed image, i.e., an image whose edges are slanted with respect to the edges of the sheet on which they are printed, is another shortcoming of prior art imagers. As a sheet moves along a printer or copier, it may be subject to air turbulence that causes misalignment. To correct the misalignment, in some printers, a side edge of the moving sheet contacts stationary guide rails along its path so the sheet straightens prior to reaching an imaging station. However, in high speed imaging, the contact time may not be sufficient to straighten the sheet and a skewed image may result.
Occasionally, grossly misaligned sheets override the guide rails, especially if they are too close to the guide rail. Rather than straightening, these sheets remain grossly misaligned and often jam in the next station, for example an imaging station or a sheet inverter. A jam in a station results in wasted time while the imager is shut down to clear the jam.
U.S. Pat. No. RE 37,007 describes a system for de-skewing in which rollers are configured to selectively drive a sheet to correct skew. The rollers are all driven by a common drive mechanism and contact with the sheet is controlled by counter rollers.